When dissimilar metals are joined to each other, joining in air is difficult in case that an oxide film is present at the surface of magnesium alloy material while oxide film at the surface of steel grows at the step of heating during joining, as in the combination of magnesium alloy material and steel.
Additionally, magnesium and steel exhibit a two-phase separation type while a solubility limit to each other is very small as seen from a Fe—Mg binary phase diagram, and therefore it is metallurgically very difficult to directly join materials having such characteristics, to each other.
Accordingly, hitherto in case of using such dissimilar metal materials as magnesium-based material and steel upon combination thereof, mechanical fastening with bolts, rivets or the like is employed (see, for example, Patent Citation 1).